Wearable data display

ABSTRACT

A transparent wearable data display having a source of collimated light, a deflector for deflecting the collimated light into a scanned beam, and a first of switchable grating elements sandwiched between first and second parallel transparent substrates, which together functioning as a first light guide. A first coupling is provided for directing the scanned beam into a first total internal reflection (TIR) light path of the first light guide along the first array column. The grating elements having diffracting and non-diffracting states, in their diffracting state deflecting light out of said light guide. The grating elements are switchable into their diffracting states one group of elements at a time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of a U.S. application Ser. No.14/794,356 filed on Jul. 8, 2015, which is a continuation of U.S.application Ser. No. 14/240,643 filed Feb. 24, 2014 which is U.S.national phase of PCT Application No. PCT/GB2012/000677 filed on Aug.22, 2012, which claims the benefit of U.S. Provisional PatentApplication No. 61/573,067 filed on Aug. 24, 2011, the disclosures ofwhich are incorporated in their entirety by reference herein.

BACKGROUND

This invention relates to a wearable display device, and moreparticularly to a wearable display using electrically switchableholographic optical elements.

There is a requirement for a compact see through data display capable ofdisplaying image content ranging from symbols and alphanumericcharacters to high-resolution pixelated images. The display should behighly transparent and the displayed image content should be clearlyvisible when superimposed over a bright background scene. The displayshould provide full colour with an enhanced colour gamut for optimaldata visibility and impact. A prime requirement is that the displayshould be as easy to wear, natural and non-distracting as possible witha form factor similar to that of ski goggles or, more desirably,sunglasses. The eye relief and pupil should be big enough to avoid imageloss during head movement even for demanding military and sportsactivities. The image generator should be compact, solid state and havelow power consumption.

The above goals are not achieved by current technology. Current wearabledisplays only manage to deliver see through, adequate pupils, eye reliefand field of view and high brightness simultaneously at the expense ofcumbersome form factors. In many cases weight is distributed in theworst possible place for a wearable display, in front of the eye. Themost common approach to providing see through relics on reflective ordiffractive visors illuminated off axis. Microdisplays, which providehigh-resolution image generators in tiny flat panels, do not necessarilyhelp with miniaturizing wearable displays because the requirement forvery high magnifications inevitably results in large diameter optics.Several ultra low form factor designs offering spectacle-like formfactors are currently available but usually require aggressivetrade-offs against field of view, eye relief and exit pupil.

The optical design benefits of diffractive optical elements (DOEs) arewell known including unique and efficient form factors and the abilityto encode complex optical functions such as optical power and diffusioninto thin layers. Bragg gratings (also commonly termed volume phasegratings or holograms), which offer the highest diffractionefficiencies, have been widely used in devices such as Head Up Displays.

It is also known that diffractive optical elements can be used toprovide virtual images for direct viewing or for viewing with the aid ofoptical systems. U.S. Pat. No. 6,052,540 by Koyama discloses aviewfinder device comprising a transmission hologram that can be locatedat a position other than in an image plane. The position of the virtualimage formed by the transmission hologram is arranged to lie at theimage plane of the optical system.

An important class of diffractive optical element known as anelectrically Switchable Bragg Gratings (SBG) is based on recording Bragggratings into a polymer dispersed liquid crystal (PDLC) mixture.Typically, SBG devices are fabricated by first placing a thin film of amixture of photopolymerisable monomers and liquid crystal materialbetween parallel glass plates. One or both glass plates supportelectrodes, typically transparent indium tin oxide films, for applyingan electric field across the PDLC layer. A Bragg grating is thenrecorded by illuminating the liquid material with two mutually coherentlaser beams, which interfere to form the desired grating structure.During the recording process, the monomers polymerize and the PDLCmixture undergoes a phase separation, creating regions densely populatedby liquid crystal micro-droplets, interspersed with regions of clearpolymer. The alternating liquid crystal-rich and liquid crystal-depletedregions form the fringe planes of the grating. The resulting Bragggrating can exhibit very high diffraction efficiency, which may becontrolled by the magnitude of the electric field applied across thePDLC layer, in the absence of an applied electric field the SBG remainsin its diffracting state. When an electric field is applied to thehologram via the electrodes, the natural orientation of the LC dropletsis changed thus reducing the refractive index modulation of the fringesand causing the hologram diffraction efficiency to drop to very lowlevels. The diffraction efficiency of the device can be adjusted, bymeans of the applied voltage, over a continuous range from essentiallyzero to near 100%. U.S. Pat. No. 5,942,157 by Sutherland et al. and U.S.Pat. No. 5,751,452 by Tanaka et al. describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices.

There is a requirement for a compact, lightweight wearable data displayproviding a high brightness, high contrast information display with ahigh degree of transparency to external light.

SUMMARY

It is an object of the present invention to provide a compact,lightweight wearable data display providing high brightness and highcontrast information visibility with a high degree of transparency toexternal light.

The objects of the invention are achieved in a first embodiment in whichthere is provided a transparent wearable data display comprising: asource; a means of collimating light from the source; a means fordeflecting the collimated light into a scanned beam; a first arraycomprising one column containing N switchable grating elementssandwiched between first and second parallel transparent substrates, thesubstrates together functioning as a first light guide; a second arraycomprising integer M columns and integer N rows of switchable gratingelements sandwiched between third and fourth parallel transparentsubstrates, the substrates together functioning as a second light guide.Transparent electrodes are applied to the first and second and the thirdand fourth substrates. Each switchable grating element has a diffractingstate and a non diffracting state. The apparatus further comprises afirst coupling means for directing the scanned beam into a first totalinternal reflection (TIR) light path between the outer surfaces of thefirst light guide along the first array column; and a second couplingmeans linking each element of the first array to the first element of arow of elements of the second array. Each element of the first arraywhen in its diffracting state directing light via the second couplingmeans into a second TIR path along a row of the second array fordirecting the first TIR light into a second TIR path between the outersurfaces of the second light guide along a row of elements of the secondarray. At least one of said electrodes of the first array is patternedinto 1×N independently switchable elements each element overlapping oneof the, first array grating elements. At least one of the electrodes ofsaid second array is patterned into M×N independently switchableelements, each element overlapping one of the second array gratingelements. In one embodiment of the invention each element of the firstarray is disposed adjacent to a first element of a row of said secondarray.

In one embodiment of the invention each switchable grating element has adiffracting state when no electric field is applied across theelectrodes sandwiching the grating element and a non diffracting statewhen a field is applied across the electrodes. Each element of the firstarray when in its diffracting state directs light from the first TIRpath into the second TIR path starting at the first element of a row ofelements of the second array and proceeding along said row. In oneembodiment of the invention the elements of said first array areswitched sequentially into their diffracting states. In one embodimentof the invention the elements of rows of the second array adjacent anelement of the first array in its diffracting state are switchedsequentially into their diffracting states. Each element of the secondarray when in its diffracting state deflects light through the fourthsubstrate.

In one embodiment of the invention each grating element of the secondarray encodes image information.

In one embodiment of the invention the outer surface of the fourthsubstrate faces a viewer of the display.

In one embodiment of the invention an element of the second array in itsdiffracting state forms an image of the information encoded within thegrating element at a predefined viewing range and an angular bearingdefined by the sweep angles of the scanned beam.

In one embodiment of the invention the substrates of the first array areparallel to the substrates of the second array.

In one embodiment of the invention the substrates of the first array areorthogonal to the substrates of the second array.

In one embodiment of the invention the first coupling means is a gratingdevice.

In one embodiment of the invention the second coupling means is agrating device abutting each of the first and second arrays.

In one embodiment of the invention each switchable grating element ofthe output array is divided into independently switchable columnsaligned orthogonally to the TIR path direction in the output array.

In one embodiment of the invention a switchable grating is a SwitchableBragg Grating (SBG).

In one embodiment of the invention the scanned beam is characterized byangular deflections in two orthogonal directions.

In one embodiment of the invention the intensity of the scanned beam ismodulated by varying the refractive index modulation of at least one ofthe switchable grating elements traversed by the beam.

In one embodiment of the invention the source of collimated lightprovides first, second and third wavelength light.

In one embodiment of the invention the source of collimated lightprovides comprises first second and third wavelength light and eachswitchable grating element is a multiplexed SBG comprising a firstgrating for diffracting first wavelength light and a second grating fordiffracting second and third wavelength light.

In one embodiment of the invention the source of collimated lightprovides comprises first second and third wavelength light and eachswitchable grating element is a multiplexed SBG comprising a firstgrating for diffracting first wavelength light, a second grating fordiffracting second wavelength light and a third grating for diffractingthird wavelength light.

In one embodiment of the invention a switchable grating elementcomprises a surface relief grating backfilled with an electricallyvariable refractive index medium.

In one embodiment of the invention each switchable grating element in atleast one of the first array and second array is divided intoindependently switchable columns aligned orthogonally to the TIR paths.The refractive index modulation of each switchable column is dynamicallycontrolled such that a predetermined amount of light is diffracted bythe switchable column through the fourth substrate.

In one embodiment of the invention N is equal to 4 and M is equal to 4.

In one embodiment of the invention the data display is one of anidentical pair of left and right eyepieces.

In one embodiment of the invention the means for providing a scannedbeam comprises: a first transparent optical substrate with an inputsurface and an output surface; a second transparent optical substratewith an input surface and an output surface; transparent electrodesapplied to the output surface of the first substrate and the inputsurface of the second substrate; an electrically variable refractiveindex layer having a planar surface and a second surface shaped toprovide an array of prisms; and a fixed refractive index layer having aplanar surface and a second surface shaped to provide an array ofprismatic cavities. The prisms and prismatic cavities have identical andopposing geometries, each prism abutting one of said prismatic cavities.The planar surface of the variable refractive index layer abuts theoutput surface of the first substrate and the planar surface of thefixed refractive index layer abuts the input surface of the secondsubstrate. The transparent electrodes are electrically coupled to avariable voltage generating means. At least one of the transparentelectrodes is patterned into independently switchable electrode elementshaving substantially the same cross sectional area as the prisms suchthat said the refractive index prisms may be selectively switched indiscrete steps from a fully diffracting to a non diffracting state by anelectric field applied across the transparent electrodes.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front elevation view of a wearable display in afirst embodiment of the invention.

FIG. 2 is a schematic, cross-sectional view of a wearable display in afirst embodiment of the invention.

FIG. 3A is a schematic front elevation view of a switchable gratingelement in a first embodiment of the invention.

FIG. 3B is a schematic cross-sectional view of a switchable gratingelement in a first embodiment of the invention.

FIG. 4A is a schematic cross-sectional view of a switchable gratingelement in a first embodiment of the invention.

FIG. 4B is a schematic front elevation view of a switchable gratingelement in a first embodiment of the invention.

FIG. 5 is a schematic plan view of an illumination source in oneembodiment of the invention.

FIG. 6 is a schematic cross-sectional view of a portion of a wearabledisplay in one embodiment of the invention.

FIG. 7 is an example of an image provided in one embodiment of theinvention.

FIG. 8 is a schematic cross-section view of a wearable display eyepiecein one embodiment of the invention.

FIG. 9 is a schematic illustration showing the subdivision of gratingelements into column shaped elements in one embodiment of the invention.

FIG. 10 is a schematic illustration showing the subdivision of gratingelements into column shaped elements in one embodiment of the invention.

FIG. 11 is a schematic cross-sectional view of a portion of a gratingelement subdivided into column elements showing the diffraction of TIRlight.

FIG. 12 is a schematic front elevation view of a wearable display in oneembodiment of the invention.

FIG. 13 is a schematic cross-sectional view of a wearable display in oneembodiment of the invention.

FIG. 14 is a schematic cross-sectional view of a wearable display in oneembodiment of the invention.

FIG. 15 is a schematic cross-sectional view of a portion of a wearabledisplay in one embodiment of the invention.

DETAILED DESCRIPTION

The invention will now be further described by way of example only withreference to the accompanying drawings.

It will apparent to those skilled in the art that the present inventionmay be practiced with some or all of the present invention as disclosedin the following description. For the purposes of explaining theinvention well-known features of optical technology known to thoseskilled in the art of optical design and visual displays have beenomitted or simplified in order not to obscure the basic principles ofthe invention.

Unless otherwise stated the term “on-axis” in relation to a ray or abeam direction refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories.

Parts of the following description will be presented using terminologycommonly employed by those skilled in the art of optical design.

It should also be noted that in the following description of theinvention repeated usage of the phrase “in one embodiment” does notnecessarily refer to the same embodiment.

In a first embodiment of the invention illustrated in the schematicfront elevation view of FIG. 1 there is provided a transparent wearabledata display comprising: an illumination source 1, a first switchablegrating array 2 and a second switchable grating array 3. The displayprovides an eyepiece that may be one of pair of identical elements usedin a binocular display. Alternatively the display may simply provide amonocular eyepiece. The illumination source which will be discussed inmore detail later in the description comprises a light source, a meansfor collimating the light; and a means for deflecting the collimatedlight into a scanned beam. Desirably, the source is a laser. The firstarray 2 comprises one column and integer number N switchable gratingelements (1×N) sandwiched between first and second parallel transparentsubstrates 25, 26. The substrates 25, 26 together function as a firstlight guide. The second array comprises M columns and N rows ofswitchable grating elements sandwiched between third and fourth paralleltransparent substrates 30, 31. The substrates 30, 31 together functionas a second light guide. The substrates 30, 31 are in orthogonal planesto those of 25, 26. Transparent electrodes which are not illustrated areapplied to the first and second and the third and fourth substrates.Advantageously the electrodes are applied to opposing faces of thesubstrates. The electrodes are configured such that the applied electricfield will be perpendicular to the substrates. The electrodes wouldtypically be fabricated from Indium Tin Oxide (ITO). In one embodimentof the invention the outer surface of the fourth substrate faces aviewer of the display.

In one embodiment of the invention the switchable grating is aSwitchable Bragg Grating (SBG).

In the embodiment of FIG. 1 the integer M is equal to 4 and N is equalto 4 in other words the first array is a 1×4 array and the second arrayis a 4×4 array. The invention docs not assume any particular value for Mor N.

The illumination source further comprises a first coupling means fordirecting the scanned beam into a first TIR light path between the outersurfaces of the first light guide along the first array column. There isfurther provided a second coupling means 16 for directing the first TIRlight into a second TIR path between the outer surfaces of the secondlight guide along a row of elements of the second array. In oneembodiment of the invention the first coupling means is a gratingdevice. In one embodiment of the invention the second coupling means isa grating device abutting each of the first and second arrays asindicated in FIG. 1.

At least one of said electrodes of the first array is patterned into 1×Nindependently switchable elements each element overlapping one of thefirst array grating elements. At least one of the electrodes of saidsecond array is patterned into M×N independently switchable elementseach element overlapping one of the second array grating elements. Againwe will assume M=4 and N=4.

In one embodiment of the invention each element of the first array isdisposed adjacent to a first element of a row of said second array. Eachswitchable grating element has a diffracting state when no electricfield is applied across the electrodes sandwiching the grating elementand a non diffracting state when a field is applied across theelectrodes. Each element of the first array when in its diffractingstate directs light from the first TIR path into the second TIR pathstarting at the first element of a row of elements of the second arrayand proceeding along said row.

In one embodiment of the invention the elements of said first array areswitched sequentially into their diffracting states. The elements ofrows of the second array adjacent an element of the first array in itsdiffracting state are switched sequentially into their diffractingstates. Each element of the second array when in its diffracting statedeflects light through the fourth substrate towards the eye of the userof the display. The rows of the second array are switched sequentially.For example, in FIG. 1 the switchable grating elements of the firstarray are indicated by 24A-24D with the element 24B being indicated asbeing in its diffracting state by a dashed line. The diffracted light102R, 102G, 102B is diffracted into the row of elements 11A-11D of thesecond array starting at element 11A. Input colour sequential red, greenblue light from the light source 40 is indicated by the rays 100R, 1000,100B. It should be noted that the light is in collimated spacedthroughout the optical process to be described. The rays are coupledinto the first array light guide into the TIR paths 101R, 101G, 101Bwhich are coupled the TIR paths indicated by the rays 102R, 102G, 102Balong the row of elements 11A-11B by the grating element 24B which is inits active state.

FIG. 2 is a schematic cross-sectional view of the display showing theinput array and the output array. The switchable grating element 24B ofthe first array and the row of switchable grating elements 11A-11B ofthe second array are illustrated. Only the red TIR path 102R isillustrated.

In one embodiment of the invention each grating element of the secondarray encodes image information. For the purpose of understanding theinvention this image information may comprise a binary dot pattern or asymbol where the dots or symbols comprise regions of material into whichgratings have been recorded surrounded by regions containing nogratings. In other words when illuminated by collimated light and in itsdiffracting state the grating element diffracts the light to form animage corresponding to said image information. In one embodiment of theinvention an element of the second array in its diffracting state formsan image of the information encoded within the grating element at apredefined viewing range and an angular bearing defined by theinstantaneous deflection angles of the scanned beam. The encodedinformation may comprise a numeric symbol or a portion of a numericsymbol. The information may be a gray level pixel. The information maybe a binary pixel or symbol characterized solely by “on” and “offstates. In other embodiments of the invention the information mayprovide a three dimensional or holographic image when the gratingelement is in its diffracting state. The invention does not assume anyparticular type of image information.

In one embodiment of the invention the source of collimated lightprovides color sequential red, green and blue illumination and eachswitchable grating element is a multiplexed Bragg grating comprising afirst grating for diffracting red light and a second grating fordiffracting blue and green light.

FIG. 3 illustrates the elements of the first array in more detail. FIG.3A is a schematic plan view of a grating element of the first array. Thegrating contains two multiplexed gratings having slant angles in the YXplane. The fringes 22A, 22B from the first grating and the fringes as23A, 23B in the second grating are indicated. The same fringes are shownin the orthogonal YZ plane in FIG. 3B.

FIG. 4 illustrates the elements of the second array in more detail. FIG.3A is a schematic cross sectional view of a switchable grating elementof the second array. The grating contains two multiplexed gratingshaving slant angles in the ZX plane. The fringes 32A, 32B from the firstgrating and the fringes as 33A, 33B in the second grating are indicated.The same fringes are shown in the orthogonal YX plane in FIG. 4B.

In a further embodiment of the invention based on the embodiment ofFIGS. 3-4 the switchable grating multiplexes separate red, green andblue diffracting Bragg gratings.

It should be apparent from consideration of FIGS. 3-4 that the inventionmay provide a monochrome display by recording a single monochromegrating within each switchable grating element. Further, since thedisplay is fundamentally transparent red green and blue diffractingarrays may be stacked to provide a colour display. However such animplementation of the invention would suffer from increased thickness.

FIG. 5 is a schematic plan view of an illumination source in oneembodiment of the invention comprising a laser module emitting red,green and blue collimated light 110R, 110G, 110B, a scanner 42 providingthe scanned beams 111R, 111G, 111B, and angular sweep expansion means 43providing the beams 112R, 112G, 112B and a grating coupler 44(essentially the first coupling means discussed above) for deflectingscanned beams 113R, 113G, 113B into a TIR path insider the light guideformed by the first array. The angular sweep expansion means maycomprise a focal system of lenses or other equivalent means known tothose skilled in the art of optical design. The invention does notassume any particular configuration of the grating coupler with respectto the first array and many alternative schemes should be apparent tothose skilled in the art of optical design. The grating coupler mayemploy any known grating technology. In a typical eyeglass where thedisplay provides left and right eyepieces it would be ergonomicallyadvantageous to integrate the illumination source within the arms of thespectacles.

In one embodiment of the invention the scanned beams are characterizedby angular deflections in two orthogonal directions which advantageouslycorrespond to the Y and X coordinate directions indicated in FIG. 1.Techniques for scanning a beam in orthogonal direction are welldocumented in the prior art.

The invention does not assume any particular beam scanning method.

Advantageously the scanner will be an electro optical device However,devices based on piezoelectric deflectors and micro electro mechanicalsystems (MEMS) may be also considered. Separate scanners may be providedfor red, green and blue light. Alternatively, a single scanner operatingon colour sequential light from separate red green and blue sources maybe used. The relative merits of such technologies in terms of scanningspeed, optical efficiency, physical robustness, size and cost should beapparent to those skilled in the art of optical design.

In one embodiment of the invention, the scanner is similar to theelectro optical micro scanner disclosed in U.S. Provisional PatentApplication No. 61/457,835 by the present inventors with filing date 16Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPICDISPLAYS. The micro scanner described in that reference comprises: afirst transparent optical substrate with an input surface and an outputsurface; a second transparent optical substrate with an input surfaceand an output surface; transparent electrodes applied to the outputsurface of the first substrate and the input surface of the secondsubstrate; an electrically variable refractive index layer having aplanar surface and a second surface shaped to provide an array ofprisms; and a fixed refractive index layer having a planar surface and asecond surface shaped to provide an array of prismatic cavities. Theprisms and prismatic cavities have identical and opposing geometries,each prism abutting one of the prismatic cavities. The planar surface ofthe variable refractive index layer abuts the output surface of thefirst substrate and the planar surface of the fixed refractive indexlayer abuts the input surface of the second substrate. The transparentelectrodes are electrically coupled to a variable voltage generatingmeans. At least one of the transparent electrodes is patterned intoindependently switchable electrode elements having substantially thesame cross sectional area as the prisms such that the refractive indexprisms may be selectively switched in discrete steps from a fullydiffracting to a non diffracting state by an electric field appliedacross the transparent electrodes.

In one embodiment of the invention the scanner scans the light intodiscrete angular steps. In an alternative embodiment of the inventionthe scanner scans the light in continuous sweeps. In one embodiment ofthe invention the intensity of the scanned beam is modulated by varyingthe refractive index modulation of at least one of the switchablegrating elements traversed by the beam. Advantageously the elements ofthe first array are used to modulate the beam. However, it will beapparent from consideration of the description and drawings that othermodulation schemes based on varying the refractive index modulation ofany of the grating elements along the beam path from the light source tothe output surface of the display may be used.

The formation of a viewable image by the display is illustrated in moredetail in FIGS. 6-7. In a typical application of the invention theviewable image is overlaid on the external scene in the manner of aHeads Up Display (HUD). FIG. 7 is a schematic cross-sectional view of aportion of the second array including the grating elements 11A-11C (seeFIGS. 1-2). The element 11C is in its diffracting state. A voltagesource for applying a voltage across each grating element is indicatedby 5 and the circuit connection to the switching electrodes across thegrating element is indicated by 51. Typically, an active matrixswitching scheme would be used to control the voltages applied to thefirst and second arrays. The TIR path of the illumination light at onepoint in the beam angular sweep is indicated by the rays 114R, 114G,114B. The light deflected out of the display at one extreme of the beamangular sweep is indicated by rays 115R, 115G, 1158 and at the otherextreme of the beam angular sweep by the rays 116R, 116G, 116B. Theoutput light forms a virtual image 111 at infinity. It should beapparent from consideration of FIG. 6 that by scanning the beam in the Xand Y directions and modulating the voltage applied across the activegrating element a symbol image such as the one illustrated in FIG. 7 maybe written. The symbol image comprises bright pixels 113 and dark pixels114. In this case the voltage modulation as indicated by the chart 52showing voltage V plotted against time t would have a binary waveformrepresented by the characteristic 53. The output light is viewed throughthe pupil 112. It should be noted that each element of the second arrayrequires a unique prescription to that all light diffracted out of theeye glass passes through an exit pupil through which the eye may observethe entire displayed image. It should be apparent that by switching thevoltage to provide grey levels and taking advantage of the colour gamutprovided by the red, green blue illumination more complex images may begenerated.

FIG. 8 is a schematic side elevation view of the display 15 in relationto the observer eye 17 in one embodiment of the invention, showing theangular extent of the display data in relation to the overall field ofview defined by the physical aperture of the display. The limiting raysdefining the overall field of view are illustrated by 115A, 115B. Therays 116A, 116B define the vertical extent of the displayed data. Intypical applications such as data displays for sports it is desirable toproject data into the lower portion of the field of view. The data mayextend across the full horizontal field if necessary.

In one embodiment of the invention each switchable grating element in atleast one of the input and output arrays is divided into independentlyswitchable columns, aligned orthogonally to the TIR paths. FIG. 9provides a front elevation of view of the elements 11A-11D of the secondarray. One column of the grating element 11A is indicated by the numeral13. The invention does not place any restrictions on the width of andnumber of column elements in a column. The refractive index modulationof each switchable column is dynamically controlled by active matrixvoltage control circuitry which is not illustrated. The refractive indexmodulation within a column can be set by the SBG recording conditions orcan be varied dynamically by modulating each column in synchronizationwith the scanning of the input light. Alternatively, a combination offixed and dynamic index modulation may be used.

The columns maximize the extraction of light from the light guide bydiffracting a predetermined amount of light from an active column out ofthe display towards the eye. Non-diffracted or zero-order light whichwould otherwise be confined to the light guide by TIR is depleted insmall steps each time the beam interacts with a column until all of thelight has been extracted. In other applications of diffractive opticalelements zero-order light is treated as a loss. However, in the presentapplication the zero order light is recycled to allow uniformout-coupling of TIR light. The diffraction efficiency of individualcolumn elements is controlled by adjusting the index modulation insynchronization with the beam scanning.

In the embodiment of the invention illustrated in FIG. 9 the gratingelements are identical in size and contain equal numbers of columns. Theuse of columns elements as described above allows the grating elementwidths to vary across an array row as in the case of the gratingelements indicated by 11E-11H in FIG. 10. The grating elements widthsmay be varied dynamically to match the extraction efficiency to the timevarying beam angle. This overcomes the problem that TIR rays withincidence angles that do not meet the exact Bragg condition (off-Braggrays) are diffracted with progressively diminishing efficiency as theangle increases up to the angular bandwidth limit, requiring morebounces before the beam or an acceptable portion of the beam is ejectedfrom the light guide.

FIG. 11 is a schematic plan view of a portion of the grating element 11Aillustrating the propagation of TIR light through the columns labelledby 13A-13C. The TIR path light inside the light guide is indicated bythe ray 102R. The diffraction efficiencies of the column elements 13A,13B, 13C for rays meeting the exact Bragg diffraction angle (referred toas on-Bragg rays) are k, k′, k″ respectively. If the TIR light isinjected into the light guide with power P₀ the power diffracted atelement 13A is kP₀ in to the ray direction 102RA. The power diffractedat the element 13B is k′(1−k)P₀ into the ray direction 102RB and so onuntil most of the beam power has been extracted and the output light isdistributed over the ray directions generally indicated by 120R. The kfactors are specified to give a fixed light output at each bounce of theTIR beam ensuring a uniform light distribution across the exit pupil ofthe display. Other light distributions maybe obtained by suitablespecification of the k-factors.

In embodiments of the invention using the switchable column principledescribed above the grating element is no longer a fixed functionalelement of the display as discussed in relation to the embodiments ofFIGS. 1-8. The term now describes the instantaneous extent of the set ofcolumns over which extraction of the light corresponds to a definedimage element (pixel) takes place. In addition to maximizing theextraction of light from the display the switchable columns principlealso allow the output put light to be distributed uniformly over theexit pupil. Furthermore, the switchable column principle allow the sizeof the exit pupil to be expanded by using a sufficiently large subset ofcolumns and matching the column prescriptions to the scanned beam raydirections. Switchable column designs for use with the present inventionmay be based on the embodiments and teachings disclosed in the U.S.Provisional Patent Application No. 61/457,835 with filing date 16 Jun.2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPICDISPLAYS which is incorporated by reference herein in its entirety.

In the embodiment of FIG. 1 the first array is orthogonal the secondarray. In an alternative embodiment of the invention illustrated inFIGS. 12-15 the substrates of the first array are parallel to thesubstrates of the second array. The advantage of such a configurationwhich will now be discussed with reference to FIGS. 12-15 is that thefirst and second arrays may share common substrates and transparentelectrode layers avoiding the fabrication problems of aligning the firstand second arrays. Again the drawings are referred to the coordinatesystem defined by the axes labelled XYZ. FIG. 12 is a schematic frontelevation view of the display showing the illumination source 1 thefirst array 6 which further comprises the elements 24A-24D and thesecond array 3. The illumination source and second array are unchangedfrom the embodiment of FIG. 1. FIG. 13 is a schematic cross-sectionalview of the first and second arrays showing the propagation of red beam.FIG. 14 is a schematic cross sectional view of the first array 6 in theZY plane. FIG. 15 is schematic cross sectional view of the first arrayin the ZX plane. The first and second arrays may abut as shown in FIG.13. In alternative embodiments of the invention the first and secondarrays may sandwich an air gap or a slab of transparent material.Turning now to FIG. 13 we see that the first and second arrays aresandwiched by the substrates 30, 31 to which transparent electrodes (notillustrated) are applied on opposing faces. The first array gratingelement 24B and the second array gratings elements 11A-11D areindicated. A passive grating device comprises a grating 29B sandwichedby substrates 28A, 28B abuts the substrate 30 overlapping the element24A. As indicated in FIG. 14 the passive grating device extends over theentire length of the first array. Although the passive grating isillustrated as four distinct elements 29A-29D in FIG. 13 the gratingwill typically have a uniform prescription along its length. Theillumination source injects colour-sequential TIR light 121R, 121G, 121Binto the light guide formed by the first array substrates which isdiffracted by the active element 24B into the ray directions 122R, 122G,122B. The passive grating diffracts the light which is totallyinternally reflected at the outer surface of the substrate 28B asrepresented by the ray paths 123R, 124R lying in the plane ZY in FIG. 12and FIG. 14. The light then proceeds to follow the TIR path 102R withinthe second array. At least one of the first or second arrays may use thecolumn element scheme described earlier.

In one embodiment of the invention a switchable grating elementaccording to the principles of the invention is a surface relief gratingbackfilled with an electrically variable refractive index medium basedon the embodiments and teachings disclosed in the U.S. ProvisionalPatent Application No. 61/457,835 with filing date 16 Jun. 2011 entitledHOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPIC DISPLAYS which isincorporated by reference herein in its entirety.

In order to ensure high transparency to external light, high contrast ofdisplayed data (i.e. high diffraction efficiency) and very low haze dueto scatter the following material characteristics are desirable. A lowindex-modulation residual grating, with a modulation not greater than0.007, is desirable. This will require a good match between therefractive index of the polymer region and the ordinary index of theliquid crystal. The material should have a high index modulationcapability with a refractive index modulation not less than 0.06. Thematerial should exhibit very low haze for HPDLC cell thicknesses in therange 2-6 micron. The HPDLC should have a good index match (to within+0.015) for glass or plastic at 630 nm. One option is 1.515 (forexample, 1737F or BK7 glasses). An alternative option would be 1.472(for example Borofloat or 7740 Pyrex glasses).

Desirably the light sources are solid-state lasers. The low etendue oflasers results in considerable simplification of the optics. LEDs mayalso be used with the invention. However, LEDs suffer from largeetendue, inefficient light collection and complex illuminator andprojection optics. A further disadvantage with regard to SBGs is thatLEDs are fundamentally unpolarized.

Any display device using lasers will tend to suffer from speckle. Thepresent invention may incorporate any type of despeckler.Advantageously, the despeckler would be based on electro-opticalprinciples. A despeckler for use with the present invention may be basedon the disclosed embodiments and teachings of PCT ApplicationNo.:US2008/001909, with International Filing Date: 22 Jul. 2008,entitled LASER ILLUMINATION DEVICE, and PCT Application No.:PCT/GB2010/002023 filed on 2 Nov. 2010 by the present inventors entitledAPPARATUS FOR REDUCING LASER SPECKLE each of which is incorporated byreference herein in its entirety. The need for a despeckler may beeliminated by using a miniature, broadband (4 nm) RGB lasers of the typesupplied by Epicrystal Inc.

Speckle arising from laser sources can be reduced by applyingdecorrelation procedures based on combining multiple sets of specklepatterns or cells from a given speckle-generating surface during thespatio-temporal resolution of the human eye. Desirably the despeckler isan electro-optical device configured to generate set of unique specklephase cells by operating on the angular or polarization characteristicof rays propagating through the device. Furthermore, the despecklerdevice may be configured in several different ways to operate on one ofmore of the phase, and ray angular characteristics of incoming light.The invention does not rely on any particular despeckler technology. Anymethod for generating and averaging speckle cells may be used with theinvention. However, solid-state methods using SBGs offer more scope forminiaturization of the illuminator module.

The optical design of a wearable display according to the principles ofthe invention will be dictated by basic geometrical considerations wellknown to those skilled in the art of optical design. The goal is tomaximize eye relief, exit pupil and field of view. Since theseparameters will impact on geometrical aberrations, dispersion and otherfactors affecting image quality some performance versus form factortrade-offs are inevitable. The preferred light source is a laser. Ifbroadband sources such as LEDs are used the design will require carefulattention to the correction of chromatic dispersion and monochromaticgeometrical aberrations. Dispersion is a problem for any DOE illuminatedby a broadband source. The degree of defocus or image blur due todispersion depends on the source spectral bandwidth and the distancefrom the DOE to the virtual image plane. Typically, the angular blur fora given wavelength and a source spectral bandwidth will be of the orderof the bandwidth divided by the wavelength. The effect of monochromaticgeometrical aberrations will depend on the field of view and pupil size.

A wearable display based on any of the above-described embodiments maybe implemented using plastic substrates. Using sufficiently thinsubstrates such embodiments could be implemented as a long clear stripapplique running from the nasal to ear ends of each eyeglass with asmall illumination module continuing laser dies, light guides anddisplay drive chip tucked into the sidewall of the eyeglass. Standardindex matched glue would be used to fix the display to the surfaces ofthe eyeglasses. The plastic substrates may be fabricated from materialssuch as polycarbonate. The transparent electrodes may be fabricated fromcarbon nanotubes (CNTs) which may be more suitable than ITO for use withflexible substrates. The display may further comprise an environmentalseal. A plastic SBG for use in the present invention may be based on theHPDLC material system and processes disclosed in a U.S. ProvisionalPatent Application No. 61/573,066 with filing date 24 Aug. 2011 by thepresent inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSEDLIQUID CRYSTAL MATERIALS AND DEVICES, which is incorporated by referenceherein in its entirety.

Although a planar display element using flat substrates has beendiscussed in the above description an eyepiece according to theprinciples of the invention may be fabricated using curved surfaces. Theinvention the invention may be used to provide a facetted surfacedisplay. In one embodiment of the invention the switchable gratings areSBGs operated in reverse mode. In reverse mode the SBG has lowdiffraction efficiency when no electric field is applied and has highefficiency when a field is applied. A reverse mode SBG for use in thepresent invention may be based on the HPDLC material system andprocesses disclosed in U.S. Provisional Patent Application No.61/573,066 with filing date 24 Aug. 2011 by the present inventorsentitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTALMATERIALS AND DEVICES which is incorporated by reference herein in itsentirety.

A key feature of all of the embodiments described above is that theyprovide the benefit of sec-through. The latter is of great importance inHead Up Displays for automobile, aviation and other transportapplications; private see-through displays such for security sensitiveapplications; architectural interior signage and many otherapplications. With the addition of a holographic brightness enhancingfilm, or other narrow band reflector affixed to one side of the display,the purpose of which is to reflect the display illumination wavelengthlight only, the see-through display can be made invisible (and hencesecure) in the opposite direction of view. Here the reflected displayillumination is effectively mirrored and therefore blocked in onedirection, making it ideal for transparent desktop display applicationsin customer or personal interview settings common in bank or financialservices settings.

Although the present application addresses wearable displays it will beclear that in any of the above embodiments the eye lens and retina maybe replaced by any type of imaging lens and a screen. Any of the abovedescribed embodiments of the invention may be used in either directlyviewed or virtual image displays. Possible applications range fromminiature displays such as those used in viewfinders to large areapublic information displays. The above described embodiments may be usedin applications where a transparent display is required. For example theinvention may be used in applications where the displayed imagery issuperimposed on a background scene such as heads up displays andteleprompters. The invention may be used to provide a display devicethat is located at or near to an internal image plane of an opticalsystem. For example any of the above described embodiments may be usedto provide a symbolic data display for a camera viewfinder in whichsymbol data is projected at an intermediate image plane and thenmagnified by a viewfinder eyepiece. It will be clear the invention maybe applied in biocular or monocular displays. The invention may also beused in a stereoscopic wearable display. Any of the above describedembodiments of the invention may be used in a rear projectiontelevision. The invention may be applied in avionic, industrial andmedical displays. There are applications in entertainment, simulation,virtual reality, training systems and sport.

SBG arrays may be fabricated using a diffractive optical mask formed ona transparent sapphire wafer. The SBG cell optical prescriptions aredefined on a cell to cell basis. The process of fabricating the SBGarray may start with the creation of a multiphase computer generatedhologram encoding the desired optical functions which is thenholographically recorded in the SBG.

It should be noted that the total internal reflection ray paths shown inthe drawings are meant to be schematic only. The number of totalinternal reflections will depend on the scrolling scheme used and theoverall geometry of the light guide formed by the display layers.Typically, in order to ensure that TIR occurs the incidence angles mustlie in the range of about 42 to about 70 degrees. It should beemphasized that the drawings are exemplary and that the dimensions havebeen exaggerated.

The method of fabricating the SBG pixel elements and the ITO electrodesused in any of the above-described embodiments of the invention may bebased on the process disclosed in the PCT Application No.:US2006/043938, claiming priority to U.S. provisional patent application60/789,595 filed on 6 Apr. 2006, entitled METHOD AND APPARATUS FORPROVIDING A TRANSPARENT DISPLAY, which is incorporated by referenceherein in its entirety.

The display devices disclosed in the present invention may employfeatures of the transparent edge lit display embodiments and teachingsdisclosed in U.S. patent application Ser. No. 10/555,661 filed 4 Nov.2005 entitled SWITCHABLE VIEWFINDER DISPLAY which is incorporated byreference herein in its entirety.

The despeckler referred to in the above description may be based on thedisclosed embodiments and teachings of PCT Application No.US2008/001909, with International Filing Date: 22 Jul. 2008, entitledLASER ILLUMINATION DEVICE, and PCT Application No.: PCT/GB2010/002023filed on 2 Nov. 2010 by the present inventors entitled APPARATUS FORREDUCING LASER SPECKLE each of which is incorporated by reference hereinin its entirety.

The optical design of the display disclosed in the present applicationmay be guided by the teachings of PCT Application No.: PCT/GB2010/001982entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY by the presentinventors which is incorporated by reference herein in its entirety.

The display disclosed in the present application may incorporate an eyetracker based on the embodiments and teachings disclosed in U.S.Provisional Patent Application No. 61/344,748 with filing date 28 Sep.2010 entitled EYE TRACKED HOLOGRAPHIC EDGE ILLUMINATED EYEGLASS DISPLAYwhich is incorporated by reference herein in its entirety.

The means for scanning collimated input light and the column arraytechnique for improving the light extraction efficiency from switchablegratings discussed above may be based on the embodiments and teachingsdisclosed in the U.S. Provisional Patent Application No. 61/457,835 withfiling date 16 Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FORAUTOSTEREOSCOPIC DISPLAYS which is incorporated by reference herein inits entirety.

The optical design of display disclosed in the present application maybe guided by the teachings of PCT Application No.: PCT/GB2010/000835with International Filing Date: 26 Apr. 2010 entitled COMPACTHOLOGRAPHIC EDGE ILLUMINATED EYEGLASS DISPLAY which is incorporated byreference herein in its entirety, which discloses eyeglass displayarchitectures based on a light guiding eyepiece in which a two dimensionarray of SBG deflectors is combined with an input beam.

The display disclosed in the present application may fabricated usingthe HPDLC material system and processes disclosed in a U.S. ProvisionalPatent Application No. 61/573,066 with filing date 24 Aug. 2011 by thepresent inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSEDLIQUID CRYSTAL MATERIALS AND DEVICES which is incorporated by referenceherein in its entirety.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

The invention claimed is:
 1. A data display comprising: a light sourceconfigured to provide collimated light modulated with image informationfor projection into a multiplicity of directions across a field of view(FOV); a first light guide supporting a first array comprising onecolumn containing M grating elements switchable between diffracting andnon-diffracting states; a second light guide supporting a second arraycomprising N columns and M rows of grating elements switchable betweendiffracting and non-diffracting states; a first coupler for directinglight from said light source into a first total internal reflection(TIR) light path within said first light guide; and a second coupler foroptically connecting each grating element of said first array to acorresponding first grating element of a row of grating elements of saidsecond array; wherein: a grating element of said first array in itsdiffracting state is configured to direct light via said second couplerinto a second TIR path along a row of said second array; a gratingelement of said second array in its diffracting state is configured todirect light out of said second light guide; said first and secondarrays are configured such that one element of said first array and oneelement of said second array are in their diffracting states and allother elements of said first and second arrays are in theirnon-diffracting states while light in a portion of said FOV Is projectedby said light source; and each combination of an element of said firstarray in its diffracting state and an element of said second array InIts diffracting state provides a path for light projected into a uniqueportion of said FOV.
 2. The data display of claim 1, wherein said lightsource is a laser scanning projector, wherein each said directioncorresponds to a unique scanning beam angle.
 3. The data display ofclaim 1, wherein said light source comprises a microdisplay and aprojection Sens, wherein each said direction corresponds to a projectedbeam angle corresponding to a unique pixel of said microdisplay.
 4. Thedata display of claim 1, wherein each said Sight guide furthercomprises; substrates sandwiching said grating elements, wherein eachsaid substrate has a transparent electrode applied to a surface of saidsubstrate, wherein at least one of said electrodes of said first arrayis patterned into 1×N independently switchable elements, each saidindependently switchable element overlapping one of said first arraygrating elements, and wherein at least one of said electrodes of saidsecond array is patterned into M×N independently switchable elements,each said independently switchable element overlapping one of saidsecond array grating elements.
 5. The data display of claim 4, whereinone of said grating elements is in said non-diffracting state when anelectric field is applied across two of said electrodes.
 6. The datadisplay of claim 1, wherein one of said grating elements of said firstarray is disposed adjacent to and in optical contact with a firstgrating element of a row of said second array.
 7. The data display ofclaim 1, wherein said first array and said second array are disposed onparallel planes.
 8. The data display of claim 1, wherein said firstarray and said second array are disposed on planes inclined at angles toeach other.
 9. The data display of claim 1, wherein each of said firstand second couplers is a grating or a prism.
 10. The data display ofclaim 1, wherein at least one at said grating elements is a gratingrecorded in a liquid crystal and polymer material system.
 11. The datadisplay of claim 1, wherein at least one of said grating elements is asurface relief grating backfilled with a refractive index medium. 12.The data display of claim 1, wherein said light source provides first,second, and third wavelength light.
 13. The data display of claim 1,wherein each of said grating elements multiplexes a first grating fordiffracting a first wavelength band of said collimated light and asecond grating for diffracting a second wavelength band of saidcollimated light.
 14. The data display of claim 1, wherein each gratingelement in at least one of said first array and said second array isdivided into independently switchable elements.
 15. The data display ofclaim 1, wherein said data display is one of a pair of left and righteyepieces.
 16. The data display of claim 1, wherein said data display iswearable.
 17. The data display of claim 1, configured to provide atransparent display.
 18. The data display of claim 1, configured to forma virtual image.
 19. The data display of claim 1, wherein at least onegrating element in said first second and second arrays has anelectrically variable index modulation.
 20. The data display of claim 1,wherein at least one grating element in said first second and secondarrays encodes image information.